254 OF THE DEEP-SEA CRUSTACEA 



As our story refers chiefly to the Decapoda, it 

 may not be amiss to give an orderly epitome of the 

 groups that compose this, the highest order of the 

 crustacean class. 



Most monographers agree in dividing the Deca- 

 poda, according to the size and finish of the abdomen 

 or **tail," into three sub-orders, namely (i) the Macrura 

 (prawns and lobsters), in which the ''tail" is at least 

 as long as the cephalothorax, and ends in a large 

 tail-fan for swimming ; (2) the Anomala (hermit-crabs), 

 whose abdomen, though it almost always ends in a 

 tail-fan of reduced proportions, is either bent or 

 coiled so as to be useless for swimming ; and (3) the 

 Brachyura (crabs), whose abdomen is tucked up under 

 the cephalothorax and never ends in a tail-fan. 



The Macrura, which are the most primitive of 

 the Decapoda, are again split into two groups, 

 namely (i) the Caridides (prawns and shrimps), in 

 which a leaf-like scale of very large dimensions is 

 attached to, and can completely conceal, the peduncle 

 of the second pair of antennae; and (2) the Astacides 

 (lobsters, crayfishes, and scorpion - lobsters), whose 

 antennal scale, when present, is not large enough to 

 entirely cover the antennal stalk. Of the Caridides, 

 three sub-groups are recognised, namely (i) the 

 Peneidea (prawns), whose third pair of thoracic legs 

 end in nippers, but are not enlarged ; (2) the Steno- 

 pidea (sponge- and zoophyte-haunting prawns) in 



